Neutron number

The neutron number (symbol N) is the number of neutrons in a nuclide.

Atomic number (proton number) plus neutron number equals mass number: Z + N = A. The difference between the neutron number and the atomic number is known as the neutron excess: D = N − Z = A − 2Z.

Neutron number is not written explicitly in nuclide symbol notation, but can be inferred as it is the difference between the two left-hand numbers (atomic number and mass).

Nuclides that have the same neutron number but different proton numbers are called isotones. This word was formed by replacing the p in isotope with n for neutron. Nuclides that have the same mass number are called isobars. Nuclides that have the same neutron excess are called isodiaphers.^[1]

Chemical properties are primarily determined by proton number, which determines which chemical element the nuclide is a member of; neutron number has only a slight influence.

Neutron number is primarily of interest for nuclear properties. For example, actinides with odd neutron number are usually fissile (fissionable with slow neutrons) while actinides with even neutron number are usually not fissile (but are fissionable with fast neutrons).

Only 58 stable nuclides have an odd neutron number, compared to 194 with an even neutron number. No odd-neutron-number isotope is the most naturally abundant isotope in its element, except for beryllium-9 (which is the only stable beryllium isotope), nitrogen-14, and platinum-195.

No stable nuclides have a neutron number of 19, 21, 35, 39, 45, 61, 89, 115, 123, or ≥ 127. There are 6 stable nuclides and one radioactive primordial nuclide with neutron number 82 (82 is the neutron number with the most stable nuclides, since it is a magic number): barium-138, lanthanum-139, cerium-140, praseodymium-141, neodymium-142, and samarium-144, as well as the radioactive primordial nuclide xenon-136, which decays by a very slow double beta process. Except 20, 50 and 82 (all these three numbers are magic numbers), all other neutron numbers have at most 4 stable nuclides (in the case of 20, there are 5 stable nuclides ³⁶S, ³⁷Cl, ³⁸Ar, ³⁹K, and ⁴⁰Ca, and in the case for 50, there are 5 stable nuclides: ⁸⁶Kr, ⁸⁸Sr, ⁸⁹Y, ⁹⁰Zr, and ⁹²Mo, and 1 radioactive primordial nuclide, ⁸⁷Rb). Most odd neutron numbers have at most one stable nuclide (exceptions are 1 (²H and ³He), 5 (⁹Be and ¹⁰B), 7 (¹³C and ¹⁴N), 55 (⁹⁷Mo and ⁹⁹Ru) and 107 (¹⁷⁹Hf and ^180mTa)). However, some even neutron numbers also have only one stable nuclide; these numbers are 0 (¹H), 2 (⁴He), 4 (⁷Li), 84 (¹⁴²Ce), 86 (¹⁴⁶Nd) and 126 (²⁰⁸Pb), the case of 84 is special, since ¹⁴²Ce is theoretically unstable to double beta decay, and the nuclides with 84 neutrons which are theoretically stable to both beta decay and double beta decay are ¹⁴⁴Nd and ¹⁴⁶Sm, but both nuclides are observed to alpha decay.^[2] (In theory, no stable nuclides have neutron number 19, 21, 35, 39, 45, 61, 71, 83–91, 95, 96, and ≥ 99) Besides, no nuclides with neutron number 19, 21, 35, 39, 45, 61, 71, 89, 115, 123, 147, ... are stable to beta decay (see Beta-decay stable isobars).

Only two stable nuclides have fewer neutrons than protons: hydrogen-1 and helium-3. Hydrogen-1 has the smallest neutron number, 0.